The invention relates to a molded part made of light metal, in particular to a crankcase for an internal combustion engine, comprising holes for receiving fastening screws with the help of which compressive strains can be exerted on the molded body in a light metal area.
As molded parts made of light metal exhibit a different thermal expansion behavior as compared to the steel-made fastening screws usually employed, the stress conditions change with the change in temperature because of compressive forces exerted on molded light metal bodies with the help of fastening screws made of steel. This may become noticeable, for example in the form of a substantial reduction of the clamping forces. So as to avoid in connection with bearing blocks of light-metal crankcases any excessive increase in the play of the bearing at higher operating temperatures, it is known (from U.S. Pat. No. 5,203,854 A) to support the crankshaft bearings with the bearing screws in gray cast inserts, which are cast into the walls of the bearing. This means that the expansion behavior is determined by the gray cast inserts, which form a substantial part of the bearing walls, and not by the adjoining light metal areas of the bearing walls, so that largely uniform conditions of thermal expansion can be expected in the zone of the crankshaft bearings and their screw connections. This has an advantageous effect on the play of the bearing or the strength of the screw joint. However, it is a drawback in connection with such known crankcases, among other things, that the weight advantage of the light metal walls is cancelled to a substantial degree because of the gray cast insert receiving the screw forces and the geometric dimensions connected therewith.
Therefore, the invention is based on the problem of realizing a molded part made of light metal, in particular a crankcase for an internal combustion engine of the type specified above, in such a manner that it is possible to create advantageous fastening conditions in spite of the different thermal expansion behavior of the light metal and the fastening screws, notably by largely exploiting the weight advantages offered by the light metal.
The problem is solved by the invention in that at least one open-pored sintered body is cast in the light metal area exposed to compressive strains into the pores of a surface layer, with infiltration of the light metal, and in that the sintered body having a lower coefficient of thermal expansion than the light metal, has a strength corresponding at least with the strength of the light metal at least after it has been combined with the infiltrated light metal.
A sintered body with a coefficient of thermal expansion that is lower than the one of the light metal is capable of altering the thermal expansion behavior of the light metal if the sintered body is joined with the light metal in a manner proof to shear, and that the composite has a strength at least corresponding with the strength of the light metal, so that the light metal is prevented from freely expanding under heat in the area of the sintered body. The shear-proof joint between the light metal and the sintered body is obtained if the light metal melt, as it is being cast preferably by a pressure casting method, infiltrates at least the pores of a surface layer of the sintered body. This permits adequately high shearing strains to build up in the finished molded part in the area of transition from the light metal to the sintered body in order to suppress any relative movement between the sintered body and the light metal. The sintered bodies, which can be employed in accordance with the given stress requirements that the molded part has to meet, can be provided in this connection with comparatively small dimensions, so that the increase in weight connected with such sintered bodies remains limited all the more so because the porosity required for the sintered bodies results in a weight reduction as compared to non-sintered inserts. Since the strength of the sintered body increases with infiltration of the light metal, the sintered body without infiltrated light metal may have a lower strength than the light metal but will nonetheless satisfy the strength requirements with respect to limitation of free expandability of the light metal if the sintered body infiltrated with light metal has the strength required for such limitation.
Particularly advantageous construction conditions can be obtained if the sintered body has a coefficient of thermal expansion adapted to the fastening screws, so that comparable conditions are obtained with respect to the strength of the screwed joint when steel parts are screwed together.
Since a shear-proof joint between the sintered body and the light metal is primarily involved in the present case by infiltrating the light metal into the pores of the sintered body, the core area of the sintered body may have a higher density than the one found in the surface area. This facilitates the infiltration of the light metal into the pores of the surface layer, on the one hand, and leads to a higher strength of the sintered bodies on the other. However, the strength of the cast, sintered bodies can be improved by reinforcements as well. Of course, such reinforcements may not permanently impair the capability of the sintered body of being infiltrated when the light metal is poured around the sintered body.